The tubes for thermally cracking hydrocarbons, for example, those for producing ethylene by thermally cracking naphtha or the like under the conditions of high temperature and high pressure (about 800.degree. to about 1100.degree. C. in temperature and up to about 5 kg/cm.sup.2 in pressure) while passing the material through the tube must have high resistance to oxidation and mechanical strength (especially creep rupture strength) at high temperatures. The cracking tube must be outstanding also in resistance to carburization because solid carbon separates out from the reaction system during operation and causes carburization by adhering to the inner surface of the tube wall and diffusing through the tube wall to deteriorate the tube material and thereby embrittle the tube.
Such reactor tubes are usually prepared from HP material (0.4C-1.75Si-25Cr-35Ni-Fe) according to ASTM standard, improved HP material (0.4C-1.75Si-25Cr-35Ni-Mo, Nb, W-Fe), etc.
From the viewpoint of improved operation efficiency and productivity, it has recently been required to conduct the thermal cracking reaction of hydrocarbons at about 1150.degree. C. or higher temperatures.
However, the conventional tube materials, although usable at temperatures of about 1100.degree. C. to about 1150.degree. C., encounter difficulty in ensuring the safety of operation at higher temperatures, rapidly deteriorating especially owing to insufficient carburization resistance to become shortened in service life.
Cracking tubes locally degraded as by carburization are repaired usually by removing the degraded portion and welding for replacement. However, if the tube has been markedly impaired in ductility after aging, the tube is liable to weld cracking due to the embrittlement of the tube material when repaired by welding. For this reason, the reactor tube material needs to have high ductility after aging.
Thus, the reactor tube material is not suitable if low in one of the foregoing characteristics, i.e., oxidation resistance, creep rupture strength at high temperatures, carburization resistance and ductility after aging.
In view of the above problem, the present invention provides a heat-resistance alloy which is outstanding in oxidation resistance, creep rupture strength at high temperatures, carburization resistance and ductility after aging for use as a tube material which is least likely to embrittle so as to ensure durability and safety of operation.